1. Technical Field
The present invention relates to a method for manufacturing a semiconductor substrate, and a method for manufacturing a semiconductor device, and particularly relates to a technique to form a silicon-on-insulator (SOI) structure on a semiconductor substrate.
2. Related Art
Field effect transistors formed on an SOI substrate have a huge benefit that semiconductor devices can be provided with low power consumption and operated with high speed due to smaller junction capacitance (capacitance between a source region and a substrate, and between a drain region and the substrate) compared to that of the transistors formed on a bulk silicon substrate.
In general, an SOI substrate is prepared in which an SOI structure is formed on the entire surface of a bulk silicon substrate. On the SOI structure, transistors are formed in order. The SOI structure is removed from an area in which the SOI structure is not needed.
A separation by bonding Si islands (SBSI) method capable of manufacturing an SOI transistor with low costs by partially forming an SOI layer on a bulk substrate is disclosed in T, Sakai et al., Second International SiGe Technology and Device Meeting, Meeting Abstract, pp. 230-231, May (2004). For example, when a related art bulk CMOS and a CMOS having an SOI structure are mixedly fabricated on a bulk silicon substrate by applying the SBSI method, first, an oxide film and a nitride film are formed on the silicon substrate in order. Then, the nitride film at a position, to which an isolation film for isolating an SOI structure area and a bulk area is formed, is removed so as to form an element isolation film (local oxidization of silicon (LOCOS) film) by a LOCOS process. Subsequently, the nitride film and the oxide film in an SOI structure area are removed. On the silicon substrate exposed after the oxide film is removed, a silicon germanium (SiGe) layer and a silicon (Si) layer are epitaxially grown. A hole to form a support (support hole) is formed there. After an oxide film for forming a support or the like is formed over them, the oxide film, the silicon layer, and the silicon-germanium layer are dry etched from the periphery of an element area shape to be achieved. When the silicon germanium layer is selectively etched with hydrofluoric-nitric acid, the silicon layer is held by the support and a cavity is formed under the silicon layer. Then, an insulation layer such as SiO2 is plugged into the cavity, thereby forming a buried oxide (BOX) layer between the silicon substrate and the silicon layer. Subsequently, a planarization treatment is performed on the substrate surface so as to expose a silicon layer, whereby an SOI structure is achieved on the bulk silicon substrate.
In etching the silicon germanium layer, the silicon layer and the film for forming the support is supported as a cantilever by the LOCOS film, which acts as a supporting part, in the vicinity of a border between the LOCOS film serving as an element isolation film and the SOI structure area.
This matter will be described with reference to FIGS. 14A through 14C as an example. FIGS. 14A through 14C show a semiconductor substrate having an SOI structure in a manufacturing process. FIG. 14A is a schematic plan view. FIG. 14B is a schematic sectional-view taken along the line A14-A′14 in FIG. 14A. FIG. 14C is a schematic sectional-view showing a subsequent process of that shown in FIG. 14B.
First, a LOCOS film 84 serving as an element isolation film is formed on a silicon substrate 81 so as to surround T2 that becomes an SOI structure area. Then, on the silicon substrate 81, a silicon germanium layer 85 and a silicon layer 86 are epitaxially grown. In this regard, on the LOCOS film 84, silicon germanium is not grown but a polysilicon layer 86a made of polycrystalline silicon is formed under a predetermined film forming condition. Next, support holes 87 are formed in the SOI structure area, and then a support forming film 88 is formed so as to bury the support holes 87, and cover a silicon layer 86 and a polysilicon layer 86a. On the support forming film 88, a photoresisit film is formed. The photoresisit film is patterned so as to form a photoresisit pattern 91 having the outer shape of the support and a shape covering the LOCOS film 84. With the photoresist pattern 91, the support forming film 88 is etched so as to form a support 88a as shown in FIG. 14B. Then, as shown in FIG. 14C, the silicon germanium layer 85 is selectively etched, whereby the silicon layer 86 is held by the support 88a and a cavity G2 is formed under the silicon layer 86. In etching the silicon germanium layer 85, a cavity is also formed under the silicon layer 86 in the vicinity of a border between the LOCOS film 84 and the SOI structure area T2, whereby the silicon layer 86 and the support forming film 88 are supported by the polysilicon layer 86a on the LOCOS film 84 as a cantilever.
Since the silicon layer 86 and the support forming layer 88 that are supported as a cantilever are extremely-thin and very brittle, they may arise a problem in that they are dropped off and adhere on other areas in a later process, thereby lowering a manufacturing yield of a semiconductor substrate 80.